The Practical Linux Hardening Guide
"Did you know all your doors were locked?" - Riddick (The Chronicles of Riddick)
I'm back, work in progress...
Table of Contents
- Introduction
- Policy Compliance
- Security Content Automation Protocol (SCAP)
- DevSec Hardening Framework
- Contributing
- External resources
- Pre install tasks
- Post install tasks
- Bootloader configuration (grub)
- Disk partitions
- Keep system updated
- Package management
- Netfilter ruleset
- TCP wrapper
- Users and groups
- System path permissions
- PAM module
- Limits
- Shadow passwords
- Linux kernel hardening
- Remove unused modules
- Secure shared memory
- IRQ balance
- Disable compilers
- Email notifications
- Backups
- External devices
- Tools
- Services
- Containers
- Deployment
- Testing configuration
Introduction
General disclaimer
The Practical Linux Hardening Guide provides a high-level overview of the security hardening GNU/Linux systems. It is not an official standard or handbook but it touches and use industry standards.
This guide also provides you with practical step-by-step instructions for building your own hardened systems and services.
A few simple rules for this project:
- this guide does not exhaust everything about Linux hardening
- it contains the different topics related to hardening (e.g. services, physical security)
- some hardening rules/descriptions can be done better
- you can think of it also as a checklist
Before you start remember:
This guide also contains my comments that may be differ from certain industry principles. If you are not sure what to do please see Policy Compliance chapter and think about what you actually do at your server.
The importance of Linux hardening
Simply speaking, hardening is the process of making a system more secure. Out of the box, Linux servers don’t come "hardened" (e.g. with the attack surface minimized). It’s up to you to prepare for each eventuality and set up systems to notify you of any suspicious activity in the future.
You need to harden your system to protect your assets as much as possible. Why it's important? Please read a great and short article that explains hardening process step by step by Michael Boelen.
The process of hardening servers involves both IT ops. and security teams and require changes to the default configuration according to industry benchmarks.
How to hardening Linux?
In my opinion you should definitely drop all non-industry policies, articles, manuals and other (especially on your production environments but also if you harden standalone home server). These lists exist to give false sense of security and they are not bases on authority standards.
We have a lot of great GNU/Linux hardening policies to provide safer operating systems compatible with security protocols. For me, CIS and the PCI-DSS compliant are about the best actual prescriptive guides.
Most of all you should use Security Benchmarks/Policies which describe consensus best practices for the secure configuration of target systems because configuring your systems in compliance eliminate the most common security fails/bugs. For example, CIS has been shown to eliminate 80-95% of known security vulnerabilities.
On the other hand e.g. STIG itself is just a complicated (for newbies difficult to implement) check-list. In my opinion ideally, real world implementation is automated via something like OpenSCAP.
You should use a rational approach, remember that more is not better. Each environment is different so security rules should all work in theory, but sometimes it not works as well.
How to read this guide?
The three levels of understanding this guide:
- read the main chapter (introduction and other sub chapters), e.g. Linux kernel hardening, it offers a general overview
- check the Useful resources for a deeper understanding
- check the Policies and on this basis, make changes
Which distribution should be used?
This guide is being written and tested on Red Hat Enterprise Linux and CentOS Linux distributions because:
- they are a free (CentOS) and open source
- they are enterprise-class
- they are stable and reliable
- they have great community support
- they are built on coherent snapshots of old packages
In the case of hardening they provide certified tools which can parse and evaluate each component of the SCAP standard.
Okay. Let's start, 3, 2, 1... STOP!
Making major changes to the direction of your systems can be risky.
The most important rule of system hardening that reasonable admins actually use is:
A production environment is the real instance of the app so all your changes make on the dev/test!
The second important rule is:
Don’t do anything that will affect the availability of the service or your system.
And the third rule is:
Make backup of entire virtual machines and important components in the middle of them.
Policy Compliance
Center of Internet Security (CIS)
The Center for Internet Security (CIS) is a nonprofit organization focused on improving public and private-sector cybersecurity readiness and response.
Please see CIS Benchmarks.
Security Technical Implementation Guide (STIG)
A Security Technical Implementation Guide (STIG) is a cybersecurity methodology for standardizing security protocols within networks, servers, computers, and logical designs to enhance overall security.
Please see Stigviewer for explore all stigs.
National Institute of Standards and Technology (NIST)
A National Institute of Standards and Technology (NIST) is a physical sciences laboratory, and a non-regulatory agency of the United States Department of Commerce.
Please see National Checklist Program (NCP).
Payment Card Industry Data Security Standard (PCI-DSS)
Payment Card Industry Data Security Standard (PCI-DSS) compliance is a requirement for any business that stores, processes, or transmits cardholder data.
In accordance with PCI-DSS requirements established a formal policy and supporting procedures for developing configuration standards for system components that are consistent with industry-accepted hardening standards like:
- Center for Internet Security (CIS)
- International Organization for Standardization (ISO)
- SysAdmin, Audit, Network, and Security (SANS) Institute
- National Institute of Standards and Technology (NIST)
Security Content Automation Protocol (SCAP)
Security Content Automation Protocol (SCAP) provides a mechanism to check configurations, vulnerability management and evaluate policy compliance for a variety of systems.
One of the most popular implementations of SCAP is OpenSCAP and it is very helpful for vulnerability assessment and also as hardening helper.
Please see SCAP Security Policies, OpenSCAP User Manual and OpenSCAP Static.
SCAP Security Guide
The auditing system settings with SCAP Security Guide project contains guidance for settings of Red Hat Enterprise Linux 7/CentOS Linux and it's validated by NIST.
You should inspect the security content of your system with oscap info module:
oscap info /usr/share/xml/scap/ssg/rhel7/ssg-rhel7-ds.xml
OpenSCAP Base
The oscap tool scans your system, validate security compliance content and generate reports and guides based on these scans.
Official OpenSCAP Base documentation say:
The command-line tool, called
oscap, offers a multi-purpose tool designed to format content into documents or scan the system based on this content. Whether you want to evaluate DISA STIGs, NIST‘s USGCB, or Red Hat’s Security Response Team’s content, all are supported by OpenSCAP.
Before use please see Using OSCAP.
# Installation:
yum install openscap-scanner
# Make a RHEL7 machine PCI-DSS compliant:
oscap xccdf eval --report report.html --profile xccdf_org.ssgproject.content_profile_pci-dss /usr/share/xml/scap/ssg/content/ssg-rhel7-ds.xml
# Make a CentOS machine PCI-DSS compliant:
oscap xccdf eval --report report.html --profile xccdf_org.ssgproject.content_profile_pci-dss /usr/share/xml/scap/ssg/content/ssg-centos7-ds.xml
SCAP Workbench
SCAP Workbench is a utility that offers an easy way to perform common oscap tasks on local or remote systems.
Before use please see Using SCAP Workbench.
# Installation:
yum install scap-security-guide scap-workbench
DevSec Hardening Framework
Security + DevOps: Automatic Server Hardening.
This project covered a lot of the things in this guide, which can be automated (e.g. setting of grub password or enforcing the permissions of the common directories).
Project: DevSec Hardening Framework + GH repository: dev-sec.
Thanks for @artem-sidorenko!
Summary
Okay, let's put together what we were talking about:
Contributing
If you find something which doesn't make sense, or one of these doesn't seem right, or something seems really stupid; please make a pull request or please add valid and well-reasoned opinions about your changes or comments.
Before add pull request please see this.
External resources
Other official hardening guides
| Type of hardening guide | Comments |
|---|---|
| STIGs Master List | |
| Arch Linux | |
| CentOS Linux | |
| Debian GNU/Linux | old guide - to update |
| Fedora Linux | old guide - to update |
| Red Hat Enterprise | |
| Slackware Linux | some data may not be available |
| Ubuntu Linux | some data may not be available |